Photo-curable ink jet ink set and ink jet recording method using the same

ABSTRACT

A photo-curable ink jet ink set includes an undercoating photo-curable ink jet ink containing a (meth)acrylic acid ester having a vinyl ether group expressed by general formula (I), and an overcoating photo-curable ink jet ink. The undercoating photo-curable ink jet ink has a higher surface tension than the overcoating photo-curable ink jet ink.

BACKGROUND 1. Technical Field

The present invention relates to a photo-curable ink jet ink set and anink jet recording method using the ink set.

2. Related Art

Various techniques are applied to recording processes for forming imageson a recording medium, such as paper, according to image data signals.Among such techniques is an ink jet technique in which images are formeddirectly on a recording medium by ejecting an ink only onto desiredimage portions, using an inexpensive apparatus. In the ink jettechnique, inks are efficiently used, and accordingly running cost canbe reduced. In addition, little noise is produced when the ink jettechnique is used, and the ink jet technique is thus advantageous as arecording method.

In order to form characters and images with high fastness to water,solvents, rubbing and so forth on the surface of a recording medium,photo-curable inks that can be cured by being irradiated with light(ultraviolet light) have been used in recent years.

Also, a recording technique performed by overcoating variousphoto-curable inks one after another is widely used. For example, acolor image may be formed by applying different color inks of cyan (C),magenta (M), yellow (Y), black (K), white (W) and other colors one ontop of another, or a clear and colorless ink may be applied to thesurface of a color image to protect the color image or to give gloss tothe color image. For these cases, a technique for forming images havingdesired quality is desired.

For example, JP-A-2006-181801 discloses an overcoating method usingenergy ray-curable ink jet recording ink compositions. In this method, aprinted coating is formed on a recording member using an energyray-curable ink jet recording color ink composition (Ia) having asurface tension Sa (mN/m), and then the coating of the color ink iscoated with a clear coating of an energy-ray curable ink jet recordingclear ink composition (Ib) having a surface tension Sb (mh/m). In thisinstance, the surface tensions of ink composition Ia, which is appliedas an undercoat, and ink composition Ib, which is applied as anovercoat, satisfy the relationships: 31.5≤Sa<35.0 or 27.0≤Sa<31.5, andSb<Sa.

However, in the method disclosed in the above-cited patent document,when the clear ink composition is applied over the color ink coatingthat has been applied as an undercoat, the line width of the clear inkbecomes very small. Thus, the clear ink composition cannot wet thesurface or spread sufficiently.

SUMMARY

An advantage of some aspects of the invention is that it provides aphoto-curable ink jet ink set including inks that exhibit a highwetting/spreading property.

The present inventors have conducted intensive research to solve theabove issue, and found that the compositions of inks, particularly thecomposition of the undercoating photo-curable ink jet ink, has arelationship with the relationship in surface tension between theundercoating photo-curable ink jet ink and the overcoating photo-curableink jet ink. More specifically, the inventors found that even if thesurface tensions of the undercoating photo-curable ink jet ink and theovercoating photo-curable ink jet ink have the relationships disclosedin the above-cited JP-A-2006-181801, the overcoating photo-curable inkjet ink can exhibit a high wetting/spreading property in a combined usewith an undercoating photo-curable ink jet ink containing a(meth)acrylic acid ester having a specific vinyl ether group.

Furthermore, the inventors found that an ink set can overcome the aboveissue which includes an overcoating photo-curable ink jet ink(hereinafter simply referred to as overcoating ink) and an undercoatingphoto-curable ink jet ink (hereinafter simply referred to asundercoating ink) containing a (meth)acrylic acid ester having aspecific vinyl ether group and having a higher surface tension than theovercoating ink.

According to an aspect of the invention, a photo-curable ink jet ink sethaving the following features is provided.

The photo-curable ink jet ink set includes an undercoating photo-curableink jet ink and an overcoating photo-curable ink jet ink. Theundercoating photo-curable ink jet ink contains a (meth)acrylic acidester having a vinyl ether group expressed by general formula (I):CH₂═CR¹—COOR¹—O—CH═CH—R³, wherein R¹ represents a hydrogen atom or amethyl group, R² represents a divalent organic residue having a carbonnumber of 2 to 20, and R³ represents a hydrogen atom or a monovalentorganic residue having a carbon number of 1 to 11. The undercoatingphoto-curable ink jet ink has a higher surface tension than theovercoating photo-curable ink jet ink.

In the photo-curable ink jet ink set, the difference in surface tensionbetween the undercoating photo-curable ink jet ink and the overcoatingphoto-curable ink jet ink may be 5 mN/m or more.

The content of the (meth)acrylic acid ester in the undercoatingphoto-curable ink jet ink may be 20% to 80% by mass relative to thetotal mass of the undercoating photo-curable ink jet ink.

The overcoating photo-curable ink jet ink may contain a (meth)acrylicacid ester having a vinyl ether group expressed by general formula (I)in a lower content than the content of the (meth)acrylic acid ester inthe undercoating photo-curable ink jet ink.

The undercoating photo-curable ink jet ink may contain an acrylicsurfactant.

The overcoating photo-curable ink jet ink may contain a siliconesurfactant.

According to another aspect of the invention, an ink jet recordingmethod using the above-described photo-curable ink jet ink set isprovided. In the method, a coating of the undercoating photo-curable inkjet ink of the ink jet ink set is formed on a recording medium. Then, acoating of the overcoating photo-curable ink jet ink of the ink jet inkset is formed on the coating of the undercoating photo-curable ink jetink.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be described with reference to the accompanyingdrawing, wherein like numbers reference like elements.

The FIGURE is a conceptual diagram of a head unit and a transport unitof a recording apparatus used in an embodiment of the invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Embodiments of the invention will now be described in detail. Theinvention is not limited to the following embodiments, and variousmodifications may be made within the scope and spirit of the invention.

The term “wetting/spreading property” mentioned therein is a property ofan ink represented by the line width of a line of an overcoating inkformed on a coating of an undercoating ink. The term “cure or curing”mentioned herein implies that an ink is solidified by polymerizing apolymerizable compound in the ink by irradiating the ink with radiation.The term “curability” mentioned herein is a property of an ink thatrepresents the degree in which the ink can be cured by responding tolight. The term “burying property” or “filling property” is a propertyof an ink that represents the degree in which a cured film of an ink cancover an underlying recording medium. A high burying property impliesthat when a cured film (image) is formed in a recorded article, theunderlying recorded medium is hidden when viewed from the image side.The term “ejection stability” is a property of an ink that representsthe degree in which the ink is stably ejected in the form of dropletsthrough nozzles without clogging the nozzles. The term “storagestability” is a property of an ink that represents the difficulty withthe change in viscosity of the ink.

In the description, the words “undercoating” and “overcoating” expressrelative positions of coating layers of inks applied one on top of theother, and may translate into “underlying” and “overlying”.

In the description, the term “(meth)acrylate” refers to at least eitheran acrylate or the corresponding methacrylate, and “(meth)acrylic”compound refers to at least either an acrylic compound or thecorresponding methacrylic compound. “(Meth)acryloyl” refers to at leasteither an acryloyl or a corresponding methacryloyl.

A “recorded article” mentioned herein refers to a thing including arecording medium and a cured film formed on the recording medium byrecording with an ink. Also, a cured film mentioned herein refers to afilm made of a cured material, and a coating of an ink mentioned hereinrefers to a film formed by applying the ink and curing the applied ink,that is, a cured coating. When a coating of an overcoating photo-curableink jet ink is formed on a coating previously formed using anundercoating photo-curable ink jet ink, whether or not the coating ofthe undercoating photo-curable ink jet ink is cured is determinedaccording to the evaluation criteria of curability described later. Acoating may form a solid image or a shaped image such as a picture, acharacter, or a pattern.

A solid image mentioned herein refers to an image defined only by pixelsin which dots are formed. The pixel refers to a minimum recording unitarea defining a recording resolution.

Photo-Curable Ink Jet Ink Set

The photo-curable ink jet ink set of an embodiment of the inventionincludes an undercoating photo-curable ink jet ink containing a(meth)acrylic acid ester having a vinyl ether group expressed by thefollowing general formula (I), and an overcoating photo-curable ink jetink. In addition, the undercoating ink has a higher surface tension thanthe overcoating ink.

The ink set may include other ink in addition to the undercoatingphoto-curable ink jet ink and the overcoating photo-curable ink jet ink.

First, the surface tension of the inks will be described. Preferably,the difference in surface tension between the undercoating ink and theovercoating ink ([surface tension of the undercoating ink]−[surfacetension of the overcoating ink]) is 5 mN/m or more, and more preferably10 mN/m or more. When the undercoating ink and the overcoating ink havesuch surface tensions, a high wetting/spreading property is exhibited.

The surface tensions of the undercoating ink and the overcoating ink arepreferably in any one of the following ranges, provided that they havethe relationship described above. Specifically, both the surfacetensions of the overcoating ink and the undercoating ink are preferablyin the range of 20 to 40 mN/m. More preferably, the surface tension ofthe overcoating ink is in the range of 20 to 30 mN/m, and the surfacetension of the undercoating ink is in the range of 25 to 40 mN/m. Anovercoating ink and an undercoating ink each having a surface tension inthe above range can be stably ejected from ink jet heads.

The surface tension of an ink depends on the composition (constituentsand their contents) of the ink, and mainly depends on surfactant and itscontent in the ink. The surfactant will be described later. The surfacetension mentioned herein is a value measured by the method that will bedescribed in Examples.

Possible constituents of the undercoating and overcoating inks will nowbe described. In the following description, the overcoating andundercoating inks may be collectively referred to as the ink(s).

Polymerizable Compound

The overcoating and undercoating inks contain a polymerizable compoundthat will be polymerized by the action of a polymerization initiatordescribed later when irradiated with radiation, thus curing the printedink.

1. Vinyl Ether-Containing (Meth)Acrylic Acid Ester

At least the undercoating ink of the ink set contains a (meth)acrylicacid ester having a vinyl ether group expressed by general formula (I):CH₂═CR¹—COOR²—O—CH═CH—R³. In general formula (I), R¹ represents ahydrogen atom or a methyl group, R² represents a divalent organic groupresidue having a carbon number of 2 to 20, and R³ represents a hydrogenatom or a monovalent organic residue having a carbon number of 1 to 11.

The (meth)acrylic acid ester having the vinyl ether group (hereinafterreferred to as vinyl ether-containing (meth)acrylic acid ester) canenhance the curability of the ink and reduce the viscosity of the ink.Also, the use of the vinyl ether-containing (meth)acrylic acid ester ismore advantageous in enhancing the curability of the ink than the casewhere a compound having the vinyl ether group and a compound having a(meth)acrylic group are each used.

In particular, when the undercoating ink contains the vinylether-containing (meth)acrylic acid ester, the overcoating ink canexhibit a high wetting/spreading property, and the undercoating inkexhibits high curability, and a high burying property on the recordingmedium.

On the other hand, the overcoating ink does not necessarily contain thevinyl ether-containing (meth)acrylic acid ester. However, it isadvantageous that the overcoating ink contains the vinylether-containing (meth)acrylic acid ester from the viewpoint ofproducing the effects described later. If the overcoating ink containsthe vinyl ether-containing (meth)acrylic acid ester, the vinylether-containing (meth)acrylic acid esters of the undercoating ink andthe overcoating ink may be the same or different.

In general formula (I), the divalent organic residue represented by R²,having a carbon number of 2 to 20, may be a substituted or unsubstitutedlinear, branched or cyclic alkylene having a carbon number of 2 to 20, asubstituted or unsubstituted alkylene having an ether bond and/or anoxygen atom of an ester bond in the molecular structure and having acarbon number of 2 to 20, or a substituted or unsubstituted divalentaromatic group having a carbon number of 6 to 11. Preferred examples ofthese divalent organic residues include alkylene groups having a carbonnumber of 2 to 6, such as ethylene, n-propylene, isopropylene, andbutylene; and alkylene groups having an oxygen of an ester bond in themolecular structure and having a carbon number of 2 to 9, such asoxyethylene, oxy n-propylene, oxyisopropylene, and oxybutylene.

In general formula (I), the monovalent organic residue represented byR³, having a carbon number of 1 to 11 may be a substituted orunsubstituted linear, branched or cyclic alkyl group having a carbonnumber of 1 to 10 or a substituted or unsubstituted aromatic grouphaving a carbon number of 6 to 11. Among these, preferred are alkylgroups having a carbon number of 1 or 2, such as methyl and ethyl, andaromatic groups having a carbon number of 6 to 8, such as phenyl andbenzyl.

If any of the organic residues is substituted, the substituent may ormay not contain a carbon atom. If the substituent contains a carbonatom, this carbon atom is counted in the carbon number of the organicresidue. Substituents containing a carbon atom include, but are notlimited to, carboxy and alkoxy. Substituents not containing a carbonatom include, but are not limited to, hydroxy and halogens.

Examples of the vinyl ether-containing (meth)acrylic acid ester include,but are not limited to, 2-vinyloxyethyl (meth)acrylate, 3-vinyloxypropyl(meth)acrylate, 1-methyl-2-vinyloxyethyl (meth)acrylate,2-vinyloxypropyl (meth)acrylate, 4-vinyloxybutyl (meth)acrylate,1-methyl-3-vinyloxypropyl (meth)acrylate, 1-vinyloxymethyl propyl(meth)acrylate, 2-methyl-3-vinyloxypropyl (meth)acrylate,1,1-dimethyl-2-vinyloxyethyl (meth)acrylate, 3-vinyloxybutyl(meth)acrylate, 1-methyl-2-vinyloxypropyl (meth)acrylate,2-vinyloxybutyl (meth)acrylate, 4-vinyloxycyclohexyl (meth)acrylate,6-vinyloxyhexyl (meth)acrylate, 4-vinyloxymethylcyclohexylmethyl(meth)acrylate, 3-vinyloxymethylcyclohexylmethyl (meth)acrylate,2-vinyloxymethylcyclohexylmethyl (meth)acrylate,p-vinyloxymethylphenylmethyl (meth)acrylate,m-vinyloxymethylphenylmethyl (meth)acrylate,o-vinyloxymethylphenylmethyl (meth)acrylate, 2-(vinyloxyethoxy)ethyl(meth)acrylate, 2-(vinyloxyisopropoxy)ethyl (meth)acrylate,2-(vinyloxyethoxy)propyl (meth)acrylate, 2-(vinyloxyethoxy)isopropyl(meth)acrylate, 2-(vinyloxyisopropoxy)propyl (meth)acrylate,2-(vinyloxyisopropoxy)isopropyl (meth)acrylate,2-(vinyloxyethoxyethoxy)ethyl (meth)acrylate,2-(vinyloxyethoxyisopropoxy)ethyl (meth)acrylate,2-(vinyloxyisopropoxyethoxy)ethyl (meth)acrylate,2-(vinyloxyisopropoxyisopropoxy)ethyl (meth)acrylate,2-(vinyloxyethoxyethoxy)propyl (meth)acrylate,2-(vinyloxyethoxyisopropoxy)propyl (meth)acrylate,2-(vinyloxyisopropoxyethoxy)propyl (meth)acrylate,2-(vinyloxyisopropoxyisopropoxy)propyl (meth)acrylate,2-(vinyloxyethoxyethoxy)isopropyl (meth)acrylate,2-(vinyloxyethoxyisopropoxy)isopropyl (meth)acrylate,2-(vinyloxyisopropoxyethoxy)isopropyl (meth)acrylate,2-(vinyloxyisopropoxyisopropoxy)isopropyl (meth)acrylate,2-(vinyloxyethoxyethoxyethoxy)ethyl (meth)acrylate,2-(vinyloxyethoxyethoxyethoxyethoxy)ethyl (meth)acrylate,2-(isopropenoxyethoxy)ethyl (meth)acrylate,2-(isopropenoxyethoxyethoxy)ethyl (meth)acrylate,2-(isopropenoxyethoxyethoxyethoxy)ethyl (meth)acrylate,2-(isopropenoxyethoxyethoxyethoxyethoxy)ethyl (meth)acrylate,polyethylene glycol monovinyl ether (meth)acrylate, and polypropyleneglycol monovinyl ether (meth)acrylate.

Among these, 2-(vinyloxyethoxy)ethyl (meth)acrylates, that is,2-(vinyloxyethoxy)ethyl acrylate and 2-(vinyloxyethoxy)ethylmethacrylate, are preferred, and 2-(vinyloxyethoxy)ethyl acrylate ismore preferred. These can reduce the viscosity of the ink, and also havehigh flash points and exhibit good curability. In particular, since2-(vinyloxyethoxy)ethyl acrylate and 2-(vinyloxyethoxy)ethylmethacrylate have simple structures and low molecular weights, theviscosity of the ink can be reduced. Examples of the2-(vinyloxyethoxy)ethyl (meth)acrylate include 2-(2-vinyloxyethoxy)ethylmethacrylate, 2-(1-vinyloxyethoxy)ethyl methacrylate,2-(2-vinyloxyethoxy)ethyl acrylate, and 2-(1-vinyloxyethoxy)ethylacrylate. In view of the curability of the ink, 2-(vinyloxyethoxy)ethylacrylate is more suitable than 2-(vinyloxyethoxy)ethyl methacrylate.

(Meth)acrylic acid esters having the vinyl ether group may be usedsingly or in combination.

In the undercoating ink, the content of the vinyl ether-containing(meth)acrylic acid ester is preferably 20% to 80% by mass, morepreferably 30% to 80% by mass, relative to the total mass (100%) of theundercoating ink. An undercoating ink containing 20% by mass or more ofthe vinyl ether-containing (meth)acrylic acid ester has a low viscosity,exhibits high curability, and allows the overcoating ink to have ahigher wetting/spreading property. Also, an undercoating ink containing80% by mass or less of the vinyl ether-containing (meth)acrylic acidester can exhibit high storage stability.

In addition, if the overcoating ink, as well as the undercoating ink,contains the vinyl ether-containing (meth)acrylic acid ester, thecontent of the vinyl ether-containing (meth)acrylic acid ester in theundercoating ink is preferably higher than that in the overcoating ink,and more preferably 10% by mass or higher. Consequently, the overcoatingink can exhibit a still higher wetting/spreading property, and thus anink set superior in various properties can be achieved.

In this instance, the vinyl ether-containing (meth)acrylic acid estercontent in the overcoating ink is not particularly limited, and ispreferably set in view of other properties as well as curability. Thereason why the overcoating ink exhibits a high wetting/spreadingproperty on a coating of the undercoating ink when the surface tensionof the undercoating ink is higher than that of the overcoating ink whilethe undercoating ink contains the vinyl ether-containing (meth)acrylicacid ester may be that a coating formed from the vinyl ether group ofthe vinyl ether-containing (meth)acrylic acid ester affects thewetting/spreading property of the overcoating ink in some way. However,other reasons may be possible.

The vinyl ether-containing (meth)acrylic acid ester can be prepared by,but not limited to, a process (process B) of esterifying a (meth)acrylicacid and a vinyl ether having a hydroxy group, a process (process C) ofesterifying a (meth)acrylic acid halide and a vinyl ether having ahydroxy group, a process (process D) of esterifying a (meth)acrylic acidanhydride and a vinyl ether having a hydroxy group, a process (processE) of esterifying a (meth)acrylic acid ester and a vinyl ether having ahydroxy group, a process (process F) of esterifying (meth)acrylic acidand a vinyl ether having a halogen, a process (process G) of esterifyingan alkali (earth) metal (meth)acrylate and a vinyl ether having ahalogen, a process (process H) of vinyl exchange between a (meth)acrylicacid ester having a hydroxy group and a carboxylic acid vinyl, or aprocess (process I) of ester-exchange between a (meth)acrylic acid esterhaving a hydroxy group and an alkyl vinyl ether.

Among these processes, process E is preferred from the viewpoint ofachieving desired results in embodiments of the invention.

2. Other Polymerizable Compounds

The inks may further contain other polymerizable compounds such as knownmonofunctional, bifunctional, trifunctional and higher polyfunctionalmonomers and oligomers. Examples of the monomers include (meth)acrylicacid, itaconic acid, crotonic acid, isocrotonic acid, maleic acid andother unsaturated carboxylic acids, their salts and esters, urethanes,amides and their anhydrides, acrylonitrile, styrene, unsaturatedpolyesters, unsaturated polyethers, unsaturated polyamides, andunsaturated urethanes. Examples of the oligomers include oligomersproduced from the above monomers, such as linear acrylic oligomers,epoxy (meth)acrylates, oxetane (meth)acrylates, aliphatic urethane(meth)acrylates, aromatic urethane (meth)acrylates, and polyester(meth)acrylates.

The polymerizable compound may contain an N-vinyl compound as anothermonofunctional or polyfunctional monomer. Examples of the N-vinylcompound include N-vinylformamide, N-vinylcarbazole, N-vinylacetamide,N-vinylpyrrolidone, N-vinylcaprolactam and acryloyl morpholine, andderivatives of these N-vinyl compounds.

(Meth)acrylic acid esters or (meth)acrylates are suitably used in theinks.

Examples of such a (meth)acrylate include monofunctional(meth)acrylates, such as isoamyl (meth)acrylate, stearyl (meth)acrylate,lauryl (meth)acrylate, octyl (meth)acrylate, decyl (meth)acrylate,isomyristyl (meth)acrylate, isostearyl (meth)acrylate,2-ethylhexyl-diglycol (meth)acrylate, 2-hydroxybutyl (meth)acrylate,butoxyethyl (meth)acrylate, ethoxydiethylene glycol (meth)acrylate,methoxydiethylene glycol (meth)acrylate, methoxypolyethylene glycol(meth)acrylate, methoxypropylene glycol (meth)acrylate, phenoxyethyl(meth)acrylate, tetrahydrofurfuryl (meth)acrylate, isobornyl(meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl(meth)acrylate, 2-hydroxy-3-phenoxypropyl (meth)acrylate, flexiblelactone-modified (meth)acrylate, t-butylcyclohexyl (meth)acrylate,dicyclopentanyl (meth)acrylate, dicyclopentenyloxyethyl (meth)acrylate,and dicyclopentenyl (meth)acrylate. Among these, phenoxyethyl(meth)acrylate is preferred.

Examples of the bifunctional (meth)acrylate include triethylene glycoldi(meth)acrylate, tetraethylene glycol di(meth)acrylate, polyethyleneglycol di(meth)acrylate, dipropylene glycol di(meth)acrylate,tripropylene glycol di(meth)acrylate, polypropylene glycoldi(meth)acrylate, 1,4-butanediol di(meth)acrylate, 1,6-hexanedioldi(meth)acrylate, 1,9-nonane diol di(meth)acrylate, neopentyl glycoldi(meth)acrylate, dimethylol-tricyclodecane di(meth)acrylate, bisphenolA ethylene oxide adduct di(meth)acrylate, bisphenol A propylene oxideadduct di(meth)acrylate, hydroxypivalic acid neopentyl glycoldi(meth)acrylate, and polytetramethylene glycol di(meth)acrylate. Amongthese, dipropylene glycol (meth)acrylate is preferred.

Examples of the trifunctional or more polyfunctional (meth)acrylatesinclude trimethylolpropane tri(meth)acrylate, ethylene oxide-modifiedtrimethylolpropane tri(meth)acrylate, pentaerythritol tri(meth)acrylate,pentaerythritol tetra(meth)acrylate, dipentaerythritolhexa(meth)acrylate, ditrimethylolpropane tetra(meth)acrylate,glycerinpropoxy tri(meth)acrylate, caprolactone-modifiedtrimethylolpropane tri(meth)acrylate, pentaerythritolethoxytetra(meth)acrylate, and caprolactam-modified dipentaerythritolhexa(meth)acrylate.

Among the above (meth)acrylates, monofunctional (meth)acrylates arepreferably used as a polymerizable compound other than the vinylether-containing (meth)acrylic acid ester. In this instance, theviscosity of the ink is reduced, and the photo-polymerization initiatorand other constituents can be sufficiently dissolved in the ink. Also,the ink can exhibit satisfactory ejection stability during ink jetrecording. It is preferable that a monofunctional (meth)acrylate and abifunctional (meth)acrylate are used in combination. The toughness, heatresistance and chemical resistance of the coating are thus enhanced.Combined use of phenoxyethyl (meth)acrylate and dipropylene glycoldi(meth)acrylate is particularly advantageous.

The above polymerizable compounds other than the vinyl ether-containing(meth)acrylic acid ester may be used singly or in combination.

The content of the polymerizable compound other than the vinylether-containing (meth)acrylic acid ester is preferably 10% to 90% bymass, more preferably 30% to 90% by mass, still more preferably 60% to90% by mass, relative to the total mass (100%) of the ink. Bycontrolling the polymerizable compound content in such a range, theviscosity and odor of the ink can be reduced, and the solubility andreactivity of the photopolymerization initiator can be increased.

Surfactant

Preferably, the inks contain a surfactant. In particular, by adding asurfactant to the overcoating ink, the inks are more likely to satisfythe relationship in which the surface tension of the undercoating ink ishigher than that of the overcoating ink. Also, by adding a surfactant tothe undercoating ink as well as the overcoating ink, the undercoatingink exhibits a good burying property on the recording medium.

The surfactant may be, but is not limited to, a silicone surfactant, anacrylic surfactant, a cationic surfactant, an anionic surfactant, anonionic surfactant, an amphoteric surfactant, or a fluorochemicalsurfactant. Preferably, a silicone surfactant or an acrylic surfactantis used. These surfactants can greatly reduce the surface tension of theink and prevent damage to components and members coming into contactwith the ink, such as the head and ink feeding mechanism of therecording apparatus.

The silicone surfactant may be, but is not limited to, a modifiedsilicone, such as a polyester-modified silicone or a polyether-modifiedsilicone. In particular, polyether-modified silicone can greatly reducethe surface tension of the ink, and consequently, the ink can exhibit asuperior wetting/spreading property on the recording medium.

Preferred example of polyether-modified silicone may bepolyether-modified polydimethylsiloxane, and preferred example ofpolyester-modified silicone may be polyester-modified polydimethylsiloxane.

Commercially available silicone surfactants include BYK-347, BYK-348,BYK-UV 3500, BYK-UV 3510, BYK-UV 3530, and BYK-UV 3570 (each product ofBYK). Among these, BYK-UV 3500 is preferred from the viewpoint ofenhancing the wetting/spreading property of the overcoating ink.

If an acrylic surfactant is used, a (meth)acrylic copolymer may be used.

Commercially available acrylic surfactants include 1970, 230, LF-1980,LF-1982 (−50), LF-1983 (−50), LF-1984 (−50), LHP-95, LHP-96, UVX-35,UVX-36, UVX-270, UVX-271, UVX-272, AQ-7120, and AQ-7130 (each product ofKusumoto Chemicals); BYK-350, BYK-352, BYK-354, BYK-355, BYK-358,BYK-380, BYK-381, and BYK-392 (each product of BYK); and Polyflow Nos.57 and 95 (each product of Kyoeisha Chemical). If an acrylic surfactantis added to the undercoating ink, BYK-350 is suitable from the viewpointof enhancing the ejection stability of the undercoating ink.

If the overcoating and undercoating inks contain a surfactant, thefollowing three cases, preferably the third case, are advantageous fromthe viewpoint of satisfying the above-described relationship of thesurface tensions.

In a first case, the overcoating ink and the undercoating ink containthe same surfactant, and the surfactant content in the overcoating inkis higher than that in the undercoating ink. The difference insurfactant content between the two inks is preferably 0.2% by mass ormore, more preferably 0.3% by mass or more.

In the first case, the overcoating ink and the undercoating ink each maycontain 0.1% to 0.5% by mass of surfactant relative to the total mass(100%) of the ink, and the surfactant content in the undercoating inkmay be 0.1% to 0.3% by mass.

In the first case, the wetting/spreading property of the overcoating inkis enhanced. In addition, since the same surfactant is used in both theovercoating ink and the undercoating ink, material cost can be reduced.

In a second case, the overcoating ink and the undercoating ink eachcontain the same or different modified silicone surfactant, and thesurfactant content in the overcoating ink is higher than that in theundercoating ink. The difference in surfactant content between the twoinks is preferably 0.2% by mass or more, more preferably 0.3% by mass ormore.

In the second case, preferably, the modified silicone surfactants in thetwo inks are each a polyether-modified silicone or a polyester-modifiedsilicone, more preferably, a polyether-modified silicone. Preferably,the same modified silicone is used from the viewpoint of reducingmaterial cost.

In the second case, the overcoating ink and the undercoating ink eachmay contain 0.1% to 0.5% by mass of surfactant relative to the totalmass (100%) of the ink, and the surfactant content in the undercoatingink may be 0.1% to 0.3% by mass. In the second case, the undercoatingink exhibits a good burying property on the recording medium.

In a third case, the overcoating ink contains a modified siliconesurfactant. In this instance, the undercoating ink does not necessarilycontain a surfactant. If the undercoating ink contains a surfactant, thesurfactant is preferably an acrylic surfactant. By adding a modifiedsilicone surfactant to the overcoating ink, the overcoating ink exhibitsa higher wetting/spreading property. The modified silicone surfactantmay be, but is not limited to, a polyether-modified silicone or apolyester-modified silicone. Polyether silicone surfactants are easilyavailable and help the undercoating ink exhibit a satisfactory buryingproperty, and are accordingly preferred.

In the third case, the overcoating ink and the undercoating ink each maycontain 0.1% to 0.5% by mass of surfactant relative to the total mass(100%) of the ink, and the surfactant content in the undercoating inkmay be 0.1% to 0.3% by mass.

In the third case, the overcoating ink exhibits superiorwetting/spreading properties. The third case is advantageous in stablyensuring a difference in surface tension between the overcoating ink andthe undercoating ink.

Photopolymerization Initiator

The inks may contain a photopolymerization initiator. Thephotopolymerization initiator is used to cure the ink on the surface ofthe recording medium by photopolymerization caused by irradiation withradiation, thus forming printed characters or images. Ultraviolet (UV)light may be used as the radiation. UV light is superior in safety, andthe use of UV light reduces the cost of light source. Anyphotopolymerization initiator can be used as long as it can produce anactive species, such as a radical or a cation, with light energy, andthus initiates a polymerization of the polymerizable compound in theink. For example, a photo-radical polymerization initiator or aphoto-cationic polymerization initiator may be used without particularlimitation, and preferably, a photo-radical polymerization initiator isused.

Examples of the photo-radial polymerization initiator include aromaticketones, acylphosphine oxide compounds, aromatic onium salt compounds,organic peroxides, thio compounds (such as thioxanthone compounds andthiophenyl group-containing compounds), hexaaryl biimidazole compounds,ketoxime ester compounds, borate compounds, azinium compounds,metallocene compounds, active ester compounds, compounds having acarbon-halogen bond, and alkylamine compounds.

Preferably, an acylphosphine oxide compound is use. Acylphosphine oxidescan enhance the curability of the ink. A thioxanthone compound may beused in addition to an acylphosphine oxide compound.

More specifically, examples of the photo-radical polymerizationinitiator include acetophenone, acetophenone benzyl ketal,1-hydroxycyclohexyl phenyl ketone, 2,2-dimethoxy-2-phenylacetophenone,xanthone, fluorenone, benzaldehyde, fluorene, anthraquinone,triphenylamine, carbazole, 3-methylacetophenone, 4-chlorobenzophenone,4,4′-dimethoxybenzophenone, 4,4′-diaminobenzophenone, Michler's ketone,benzoin propyl ether, benzoin ethyl ether, benzyldimethyl ketal,1-(4-isopropylphenyl)-2-hydroxy-2-methylpropane-1-one,2-hydroxy-2-methyl-1-phenylpropane-1-one, thioxanthone,diethylthioxanthone, 2-isopropylthioxanthone, 2-chlorothioxanthone,2-methyl-1-[4-(methylthio)phenyl]-2-morpholino-propane-1-one,bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide,2,4,6-trimethylbenzoyl-diphenyl-phosphine oxide,2,4-diethylthioxanthone, andbis-(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide.

Commercially available photo-radical polymerization initiators includeIRGACURE 651 (2,2-dimethoxy-1,2-diphenylethane-1-one), IRGACURE 184(1-hydroxycyclohexyl phenyl ketone), DAROCUR 1173(2-hydroxy-2-methyl-1-phenyl-propane-1-one), IRGACURE 2959(1-[4-(2-hydroxyethoxy)phenyl]-2-hydroxy-2-methyl-1-propane-1-one),IRGACURE 127(2-hydroxy-1-{4-[4-(2-hydroxy-2-methyl-propionyl)benzyl]phenyl]-2-methylpropane-1-one),IRGACURE 907(2-methyl-1-(4-methylthiophenyl)-2-morpholinopropane-1-one), IRGACURE369 (2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-1-butanone),IRGACURE 379(2-(dimethylamino)-2-[(4-methylphenyl)methyl]-1-[4-(4-morpholinyl)phenyl]-1-butanone),DAROCUR TPO (2,4,6-trimethylbenzoyldiphenylphosphine oxide), IRGACURE819 (bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide), IRGACURE 784(bis(75-2,4-cyclopentadiene-1-yl)-bis(2,6-difluoro-3-(1H-pyrrole-1-yl)phenyl)titanium), IRGACURE OXE 01 (1-[4-(phenylthio)phenyl]-1,2-octanedione2-(O-benzoyloxime)), IRGACURE OXE 02(1-[9-ethyl-6-(2-methylbenzoyl)-9H-carbazole-3-yl]ethanone 0-acetyloxime), IRGACURE 754 (mixture of oxyphenylacetic acid, and2-[2-oxo-2-phenylacetoxyethoxy]ethyl ester and 2-(2-hydroxyethoxy)ethylester) (each produced by BASF); Speedcure TPO and Speedcure DETX(2,4-diethylthioxanthone), Speedcure ITX (2-isopropylthioxanthone) (eachproduced by Lambson); KAYACURE DETX-S (2,4-diethylthioxanthone)(produced by Nippon Kayaku Co., Ltd.); Lucirin TPO, LR 8893, and LR 8970(each produced by BASF); and Ubecryl P36 (produced by UCB).

The above photopolymerization initiators may be used singly or incombination.

From the viewpoint of increasing the UV curing speed to enhance thecurability, and preventing stain of the photopolymerization initiator orundissolved part of the photopolymerization initiator, thephotopolymerization initiator content is preferably 3% to 20% by massrelative to the total mass (100%) of the ink.

Polymerization Inhibitor

The inks may contain a polymerization inhibitor. By addition apolymerization inhibitor to the ink, the polymerizable compound in theink can be prevented from polymerizing before curing, and thus thestorage stability of the ink can be enhanced.

Examples of the polymerization inhibitor include, but are not limitedto, phenols such as p-methoxyphenol, cresol, t-butyl catechol,3,5-di-t-butyl-4-hydroxytoluene, 2,2′-methylenebis(4-methyl-6-t-butylphenol),2,2′-methylene-bis(4-ethyl-6-butylphenol), and4,4′-thio-bis(3-methyl-6-t-butylphenol), hindered amine, hydroquinonemonomethyl ether (MEHQ), and hydroquinone.

The above polymerization inhibitors may be used singly or incombination. The polymerization inhibitor content is not particularlylimited, and it depends on the proportion to the other constituents.

Coloring Material

The inks may contain a coloring material. The coloring material may beeither or both of a pigment and a dye.

Pigment

The use of a pigment enhances the resistance to light of the ink. Thepigment may be selected from inorganic pigments or organic pigments.

Exemplary inorganic pigments include carbon blacks (C. I. Pigment Black7), such as furnace black, lampblack, acetylene black, and channelblack; iron oxide; and titanium oxide.

Exemplary organic pigments include insoluble azo pigments, such asinsoluble azo pigments, condensed azo pigments, azo lake, and chelateazo pigments; polycyclic pigments, such as phthalocyanine pigments,perylene and perinone pigments, anthraquinone pigments, quinacridonepigments, dioxane pigments, thioindigo pigments, isoindolinone pigments,and quinophthalone pigments; dye chelates, such as basic dye chelatesand acid dye chelates; dye lakes, such as basic dye lakes and acid dyelakes; and nitro pigments, nitroso pigments, aniline black, and daylightfluorescent pigments.

Carbon blacks may be used for black inks. Examples of such a carbonblack include No. 2300, No. 900, MCF 88, No. 33, No. 40, No. 45, No. 52,MA7, MA8, MA100, and No. 2200B (each produced by Mitsubishi ChemicalCorporation); Raven 5750, Raven 5250, Raven 5000, Raven 3500, Raven1255, and Raven 700 (each produced by Carbon Columbia); Regal 400R,Regal 330R, Regal 660R, Mogul L, Monarch 700, Monarch 800, Monarch 880,Monarch 900, Monarch 1000, Monarch 1100, Monarch 1300, and Monarch 1400(each produced by Cabot); and Color Black FW1, Color Black FW2, ColorBlack FW2V, Color Black FW18, Color Black FW200, Color Black S150, ColorBlack S160, Color Black S170, Printex 35, Printex U, Printex V, Printex140U, Special Black 6, Special Black 5, Special Black 4A, and SpecialBlack 4 (each produced by Degussa).

Pigments that can be used for a white ink include C. I. Pigment whites6, 18, and 21.

Pigments that can be used in a yellow ink include C. I. Pigment Yellows1, 2, 3, 4, 5, 6, 7, 10, 11, 12, 13, 14, 16, 17, 24, 34, 35, 37, 53, 55,65, 73, 74, 75, 81, 83, 93, 94, 95, 97, 98, 99, 108, 109, 110, 113, 114,117, 120, 124, 128, 129, 133, 138, 139, 147, 151, 153, 154, 167, 172 and180.

Pigments that can be used in a magenta ink include C. I. Pigment Reds 1,2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 14, 15, 16, 17, 18, 19, 21, 22, 23,30, 31, 32, 37, 38, 40, 41, 42, 48(Ca), 48(Mn), 57(Ca), 57:1, 88, 112,114, 122, 123, 144, 146, 149, 150, 166, 168, 170, 171, 175, 176, 177,178, 179, 184, 185, 187, 202, 209, 219, 224 and 245, and C. I. PigmentViolets 19, 23, 32, 33, 36, 38, 43 and 50.

Pigments that can be used in a cyan ink include C. I. Pigment Blues 1,2, 3, 15, 15:1, 15:2, 15:3, 15:34, 15:4, 16, 18, 22, 25, 60, 65 and 66,and C. I. Vat Blues 4 and 60.

Pigments that can be used for colors other than magenta, cyan and yellowinclude C. I. pigment greens 7 and 10, C. I. Pigment Browns 3, 5, 25,and 26, and C. I. Pigment Oranges 1, 2, 5, 7, 13, 14, 15, 16, 24, 34,36, 38, 40, 43, and 63.

The above pigments may be used singly or in combination.

If a pigment is used in the ink, the average particle size of thepigment is preferably 300 nm or less, and more preferably 50 to 200 nm.Such a pigment can be stably dispersed in the ink, and the ink can bereliable in, for example, ejection stability, and can form high-qualityimages. The average particle size mentioned herein is a value measuredby dynamic light scattering.

Dye

A dye may be used as a coloring material. Any dye, such as acid dyes,direct dyes, reactive dyes, and basic dyes, may be used withoutparticular limitation. Examples of the dye include C. I. Acid Yellows17, 23, 42, 44, 79 and 142, C. I. Acid Reds 52, 80, 82, 249, 254 and289, C. I. Acid Blues 9, 45 and 249, C. I. Acid Blacks 1, 2, 24 and 94,C. I. Food Blacks 1 and 2, C. I. Direct Yellows 1, 12, 24, 33, 50, 55,58, 86, 132, 142, 144 and 173, C. I. Direct Reds 1, 4, 9, 80, 81, 225and 227, C. I. Direct Blues 1, 2, 15, 71, 86, 87, 98, 165, 199 and 202,C. I. Direct Blacks 19, 38, 51, 71, 154, 168, 171 and 195, and C. I.Reactive Reds 14, 32, 55, 79 and 249, and C. I. Reactive Blacks 3, 4 and35. The above dyes may be used singly or in combination.

The coloring material content is preferably 0.5% to 20% by mass relativeto the total mass (100% by mass) of the ink. Such a content results in ahigh hiding power and a high color reproduction.

Additives

The inks may further contain additives (constituents). Possibleadditives include, but are not limited to, known dispersants,polymerization promoters, penetration enhancers, and wetting agents(moisturizing agents). In addition, other additives may be added, suchas a fixing agent, a fungicide, a preservative, an antioxidant, anultraviolet absorbent, a chelating agent, and a thickener.

In the use of the photo-curable ink jet ink set of the above-describedembodiment, when the overcoating ink is applied on a coating of theundercoating ink, the line width of the overcoating ink is not reduced.Thus, the overcoating ink exhibits a good wetting/spreading property. Inaddition, both the overcoating ink and the undercoating ink of thephoto-curable ink jet ink set exhibit high curability and ejectionstability, and the undercoating ink exhibits satisfactory buryingproperty on the recording medium. As described above, in thephoto-curable ink jet ink set, the undercoating ink and the overcoatingink are interactive in terms of composition and physical properties. Inother words, for controlling the compositions and physical properties ofone of the undercoating and overcoating inks, it is required that thecompositions and physical properties of both inks be taken into account.

Recording Medium

The photo-curable ink jet ink set of an embodiment of the invention isused for forming a recorded article by an ink jet recording methoddescribed later, in which the inks of the ink set are ejected onto arecording medium. The recording medium may be absorbent ornon-absorbent. The ink jet recording method described later can bewidely applied to various recording media from a non-absorbent recordingmedium into which inks cannot easily penetrate to an absorbent recordingmedium into which inks easily penetrate.

Absorbent recording media include, but are not limited to, plane papersuch as electrophotographic paper having high permeability to inks, inkjet paper having an ink absorbing layer containing silica particles oralumina particles or an ink absorbing layer made of a hydrophilicpolymer such as polyvinyl alcohol (PVA) or polyvinyl pyrrolidone (PVP),and art paper, coat paper and cast-coated paper that are used forordinary offset printing and have relatively low permeability to inks.

Non-absorbent recording media include, but are not limited to, plasticfilms, sheets and plates, such as those of polyvinyl chloride,polyethylene, polypropylene, and polyethylene terephthalate (PET); metalplates, such as those of iron, silver, copper, and aluminum;metal-coated metal plates or plastic films formed by vapor-depositingthose metals on a metal plate or plastic film; and alloy plates, such asthose of stainless steel and brass.

Ink Jet Recording Method

An ink jet recording method according to an embodiment of the inventionwill be described below. In the ink jet recording method, an ink setaccording to an embodiment of the invention is used.

Ink Jet Recording Apparatus for Ink Jet Recording Method

An ink jet recording apparatus (hereinafter simply referred to as therecording apparatus) used in the ink jet recording method will now bedescribed. The recording apparatus includes a recording control section,and operates in a first recording mode or a second recording mode.

In the first recording mode, a coating of the overcoating ink is formedon a coating of the undercoating ink by overcoating the coating of theundercoating ink with the overcoating ink.

In the second recording mode, a coating of the undercoating ink isformed on a coating of the overcoating ink by overcoating the coating ofthe overcoating ink with the undercoating ink.

The recording control section controls the amount per dot of ink in asolid image of the undercoating ink in the first and second recordingmodes. The recording apparatus may have the following structure.

The FIGURE schematically shows head units and a transport unit of arecording apparatus applicable to the ink jet recording method of thepresent embodiment of the invention. A recording medium S in the form ofcontinuous sheet is transported in a transport direction from anupstream side to a downstream side in association with the rotation ofan upstream roller 12A, a transport drum 12, and a downstream roller12B. The head units include a first white head unit 30A (located at theleftmost in the FIGURE) that ejects a white ink W, a yellow heat unit30B that ejects a yellow ink Y, a magenta head unit 30C that ejects amagenta ink M, a cyan head unit 30D that ejects a cyan ink C, a blackhead unit 30E that ejects a black ink K, and a second white head unit30A (located at the rightmost in the FIGURE) that ejects a white ink W.An irradiation unit 40 includes irradiation portions 42 a, 41 a, 41 b,41 c, 41 d and 42 b that are provided for each head unit at thedownstream side of the corresponding head unit, and an irradiationportion 43 provided at the end of the transport. The recording apparatusmay be in the structure shown in FIG. 2 disclosed in JP-A-2010-208218.

In the first recording mode, first, the recording medium is transportedto the position opposing the first white head unit 30A, and the firsthead unit 30A elects an undercoating ink to the recording medium S. Theejected undercoating ink is irradiated from the irradiation portion 42 ato form a coating of the undercoating ink. Subsequently, the recordingmedium S is transported to the portion opposing each head unit from oneto another, and an overcoating ink is ejected from at least one of theyellow head unit 30B, the magenta head unit 30C, the cyan head unit 30Dand the black head unit 30E onto the coating of the undercoating ink.The overcoating ink is irradiated from the irradiation portion at thedownstream side of the head unit that has ejected the overcoating ink.Finally, irradiation portion 43 irradiates the overcoating ink to form acoating. In the first recording mode, the second white head unit 30A andthe irradiation portion 42 b are not used.

On the other hand, in the second recording mode, the recording medium Sis transported to the portion opposing each head unit from one toanother, and an overcoating ink is ejected, in the order in which therecording medium is transported, from at least one of the yellow headunit 30B, the magenta head unit 30C, the cyan head unit 30D and theblack head unit 30E onto the recording medium S, and the ejectedovercoating ink is irradiated from the irradiation portion at thedownstream side of the head unit that has ejected the overcoating ink toform a coating of the overcoating ink. Subsequently, the second whitehead unit 30A ejects an undercoating ink onto the coating of theovercoating ink, followed by irradiation from the irradiation portion 42b. Finally, irradiation from the irradiation portion 43 is performed toform a coating. In the second recording mode, the first white head unit30A and the irradiation portion 42 a are not used.

Preferably, when a solid coating is formed with the undercoating ink,the recording control section controls so that the amount per dot of theundercoating ink in the second recording mode is more than 10% larger,preferably 10% to 50% larger, than that in the first recording mode.Thus, the undercoating ink in the second recording mode can exhibitsubstantially the same wetting/spreading properties as in the firstrecording mode. The dot mentioned herein refers to a mark of an inkformed in a pixel, which is a minimum recording unit area defining arecording resolution.

The recording apparatus does not necessarily have the function forperforming the second recording mode. In this instance, the second whitehead unit 30A and the irradiation portion 42 b may be omitted. The inkthat forms a coating afterward, that is, the overcoating ink in thefirst recording mode or the undercoating ink in the second recordingmode, may be cured only by irradiation from at least one of theirradiation portions (41 a to 41 d and 42 b) provided at the downstreamsides of the respective head units and the irradiation portion 43. Ifthe ink is cured by the irradiation portion at the downstream side ofthe corresponding head unit, the irradiation portion 43 at the end ofthe transport may be omitted.

Overcoating Ink and Undercoating Ink

In the present embodiment, in the first recording mode, the undercoatingink is an ink that will form a coating on a recording medium before theovercoating ink, and the overcoating ink is an ink that will form acoating on the coating of the undercoating ink on the recording medium.One of the overcoating ink and the undercoating ink may be a main ink,and the other may be an auxiliary ink. For example, an ink that forms avisible coating of, for example, a color ink, can be used as the mainink. Also, a white ink, a clear ink, a metallic ink or any other specialink, or a functional ink for improving the properties of the main ink,such as adhesion, wetting/spreading property, and rub fastness, may beused as the auxiliary ink.

Both the overcoating ink and the undercoating ink may be used as maininks, or used as auxiliary inks. However, it is preferable that one ofthe overcoating ink and the undercoating ink be used as the main inkwhile the other is used as the auxiliary ink from the viewpoint ofexpanding the application of the recorded article and improving theproperties of the main ink. If one of the inks is used as the main inkwhile the other is used as the auxiliary ink, which ink is used as themain ink can be determined according to the properties of theovercoating ink and the undercoating ink. Therefore, the undercoatingink is not limited to a white ink although, in the FIGURE, the head thatejects the undercoating ink in the first recording mode is the firstwhite head unit 30A.

In the second recording mode, the line width of the undercoating ink onthe coating of the overcoating ink is reduced, and accordingly, theburying property of the solid image of the undercoating ink is reduced.Therefore, the amount per dot of ink is varied between the firstrecording mode and the second recording mode. By increasing the amountper dot of the undercoating ink, the undercoating ink can exhibit asatisfactory burying property.

Ink Jet Recording Method using Ink Jet Recording Apparatus

The ink jet recording method according to an embodiment of the inventionincludes a first recording operation in which a coating of theundercoating ink of the above embodiment is formed on a recordingmedium, and a second recording operation in which an image is formed byovercoating the coating of the undercoating ink with the overcoating inkof the above embodiment.

Each of the first and second recording operations includes the step ofejecting an ink to the recording medium (ejection step) and the step ofirradiating the ejected ink with UV light to cure the ink (irradiationstep). The cured inks form an image, that is, a cured ink coating. Thesesteps will be described below.

Ejection Step

In the ejection step, an ink is ejected to the recording medium. Theundercoating ink is ejected onto a recording medium, and the overcoatingink is ejected onto the coating of the undercoating ink. Preferably, theink to be ejected has a viscosity of 15 mPa·s or less, more preferably12 mPa-s or less. An ink having a viscosity in such a range can beejected at room temperature or without being heated. Preferably, thetemperature of the ink to be ejected is 20 to 30° C. Alternatively, theink may be heated to a predetermined temperature so that the viscosityof the ink is adjusted to a level suitable to be ejected. Thus the inkis stably ejected.

The undercoating ink and overcoating ink used in the present embodimentare photo-curable and have higher viscosities than general aqueous inks.Accordingly, the viscosities of the inks are considerably varied bytemperature changes of the ink to be ejected. Such variation inviscosity of the ink significantly affects the size of ink droplets andthe ejection speed of the droplets, and may result in a degraded imagequality. Accordingly, it is preferable that the ink is kept at aconstant temperature while being ejected.

Curing Step

In the curing step, the ink ejected to and deposited (landed) on therecording medium is cured by irradiation with UV light. Morespecifically, the photopolymerization initiator in the ink is decomposedto produce an initiation species, such as a radical, an acid or a base,by the irradiation with light (UV light), and the initiation speciesinduces the polymerization of the polymerizable compound. Alternatively,the polymerizable compound may initiate a polymerization reaction byirradiation with UV light. At this time, if a sensitizing dye is presentin the ink together with the polymerization initiator, the sensitizingdye is excited by absorbing UV light. The excited sensitizing dye thencomes into contact with the polymerization initiator and promotes thedecomposition of the polymerization initiator, and thus, a highlysensitive curing reaction can be performed.

A mercury lamp or a solid laser is generally used as an UV light source.For curing photo-curable ink jet inks, a mercury lamp or a metal halidelamp is widely used. However, since the light source is desired to bemercury-free from the viewpoint of environmental protection, the use ofGaN-based semiconductor UV emission devices is advantageous in industryand environment. Also, ultraviolet light-emitting diodes (UV-LEDs) andultraviolet laser diodes (UV-LDs), which are small and inexpensive andhave long life and high efficiency, are considered to a useful lightsource for photo-curable ink jet recording. UV-LEDs are advantageous.

For example, a UV-LED having an emission peak at a wavelength in therange of 350 to 420 nm is advantageously used. The power of such aUV-LED is easy to increase. In this instance, an ink set ofphoto-curable ink jet inks that can be cured at an irradiation energy of200 mJ/cm² or less is suitably used. Thus, low-cost, high-speed printingcan be achieved. Such photo-curable inks contain at least either aphotopolymerization initiator that will be decomposed by irradiationwith UV light having a wavelength in the above range or a polymerizablecompound that will initiates a polymerization by irradiation with UVlight having a wavelength in the above range.

According to the ink jet recording method of the present embodiment, thephoto-curable ink jet ink set can exhibit superior properties. Theovercoating ink exhibits a satisfactory wetting/spreading property; boththe overcoating ink and the undercoating ink exhibit high curability andejection stability; and the undercoating ink exhibits satisfactoryburying property on the recording medium.

EXAMPLES

The above-described embodiments of the invention will now be furtherdescribed in detail with reference to, but without limitation to,Examples.

Materials Used

The following materials were used in the Examples and ComparativeExamples.

Pigment

-   -   C. I. Pigment Blue 15:3 (Phthalocyanine Blue produced by DIC,        hereinafter abbreviated as PB 15:3)    -   C. I. Pigment White 6 (titanium oxide produced by Tayca Corp.,        hereinafter abbreviated as PW6)        Polymerizable Compound        1. Vinyl Ether-Containing (Meth)acrylic Acid Ester    -   VEEA (2-(2-vinyloxyethoxy)ethyl acrylate, produced by Nippon        Shokubai Co., Ltd., hereinafter abbreviated as VEEA)        2. Other Polymerizable Compounds    -   Biscoat #192 (product name of phenoxyethyl acrylate (hereinafter        abbreviated as PEA) produced by Osaka Organic Chemical Industry        Ltd.)    -   NK ESTER APG-100 (Product of dipropylene glycol diacrylate        (hereinafter abbreviated as DPGDA) produced by Shin-Nakamura        Chemical)        Polymerization Inhibitor    -   MEHQ (hydroquinone monomethyl ether, produced by Kanto Chemical)        Surfactant    -   BYK-UV 3500 (silicone surfactant product of BYK, hereinafter        abbreviated as UV 3500)    -   BYK-UV 3570 (silicone surfactant product of BYK, hereinafter        abbreviated as UV 3570)    -   BYK-350 (acrylic surfactant product of BYK, hereinafter        represented by BYK 350)    -   BYK-381 (acrylic surfactant product of BYK, hereinafter        represented by BYK 381) Photopolymerization Initiator    -   IRGACURE 819 (product of BASF, solid content: 100%, hereinafter        abbreviated as 819)    -   DAROCUR TPO (product of BASF, solid content: 100%, hereinafter        abbreviated as TPO)    -   KAYACURE DETX-S (product of Nippon Kayaku, solid content: 100%,        hereafter abbreviated as DETX)        Preparation of Overcoating Ink and Undercoating Ink

An overcoating cyan ink and an undercoating white ink were prepared bymixing the above constituents in the proportions (percent by mass) shownin Table 1 or 2 and stirring the mixture with a high-speed water-coolingagitator.

Table 1 shows the compositions of the prepared overcoating inks, and inwhich overcoats 1, 2, 3 and 4 represents overcoating inks 1, 2, 3 and 4,respectively. Similarly, Table 2 shows the compositions of the preparedundercoating inks, and in which undercoats 1, 2, 3, 4, 5, 6 and 7represents overcoating inks 1, 2, 3, 4, 5, 6 and 7, respectively.

TABLE 1 Overcoat 1 Overcoat 2 Overcoat 3 Overcoat 4 VEEA 20 — 20 20 PEA35 45 35 35 DPGDA 31.6 41.6 31.6 31.6 819 4 4 4 4 TPO 4 4 4 4 DETX 2 2 22 MEHQ 0.2 0.2 0.2 0.2 UV3500 0.2 0.2 — — UV3570 — — — 0.2 BYK350 — —0.2 — PB15:3 3 3 3 3 Total 100.0 100.0 100.0 100.0

TABLE 2 Undercoat Undercoat Undercoat Undercoat Undercoat UndercoatUndercoat 1 2 3 4 5 6 7 VEEA 30 30 30 30 30 — 70 PEA 15 15 15 15 15 30 1DPGDA 26.6 26.4 26.8 26.6 26.6 41.6 0.4 819 4 4 4 4 4 4 4 TPO 4 4 4 4 44 4 DETX 2 2 2 2 2 2 2 MEHQ 0.2 0.2 0.2 0.2 0.2 0.2 0.2 UV3500 — — — 0.2— — — BYK350 0.2 0.4 — — — 0.2 0.4 BYK381 — — — — 0.2 — — PW6 18 18 1818 18 18 18 Total 100.0 100.0 100.0 100.0 100.0 100.0 100.0

Examples 1 to 7, Comparative Examples 1 to 5

Ink sets were prepared in the combinations of the overcoating ink andthe undercoating ink shown in Tables 3 and 4. In Tables 3 and 4, thesurface tensions γ (mN/m) of the overcoating ink and the undercoatingink are measurements obtained by the Wilhelmy method using a surfacetension meter CBVP-Z (product of Kyowa Interface Science) at roomtemperature and normal pressure.

Evaluation

The wetting/spreading property, burying property, ejection stability andcurability of the ink sets prepared in Examples and Comparative Exampleswere evaluated.

1. Wetting/Spreading Property

The undercoating ink was ejected onto a recording medium (PET50A,manufactured by Lintec Corporation) at 720 dpi by 720 dpi and 15 ng/dot.The ejected ink was then cured to form a cured film. Then, a line ofdots (dots in a line) with 720 dpi was formed on the cured film usingthe overcoating ink at 10 ng/dot.

Before ejection, the viscosity of each ink was adjusted to 10 mPa·s byadjusting the temperature of the ink. For evaluation, the line width (X)of the cured line of the overcoating ink formed on the cured film of theundercoating ink was measured. The evaluation criteria were as follows,and the results are shown in Table 3 and 4:

-   -   A: 70 μm≤X    -   B: 65 μm≤X<70 μm    -   C: 60 μm≤X<65 μm    -   D: X<60 μm        2. Burying Property

The undercoating ink was ejected onto a recording medium (PET50A) at 720dpi by 720 dpi and 10 ng/dot, followed by curing.

Before ejection, the viscosity of the ink was adjusted to 10 mPa-s byadjusting the temperature of the ink. The resulting cured film wasvisually observed for the burying property of the undercoating ink.

The evaluation criteria were as follows, and the results are shown inTable 3 and 4:

-   -   A: when the sample was visually viewed from a height of 30 cm        from the recording medium, the surface of the recording medium        was covered with the cured film and striped marks of the        underlying recording medium were not observed.    -   B: when the sample was visually viewed from a height of 30 cm        from the recording medium, striped marks of the recording medium        were observed.        3. Ejection Stability

The ejection stabilities of the overcoating ink and the undercoating inkwere evaluated. The overcoating ink and the undercoating ink were eachcontinuously ejected for 60 minutes from a head having 180 nozzles. Thusthe ejection stabilities of the overcoating and undercoating inks wereevaluated. The evaluation criteria were as follows, and the results areshown in Table 3 and 4:

-   -   A: nozzle fault (ejection failure) did not occur during        continuous ejection.    -   B: nozzle fault (ejection failure) occurred during continuous        ejection.        4. Curability

The curabilities of the overcoating ink and the undercoating ink wasevaluated. The nozzles of the nozzle lines of each ink were charged withthe corresponding ink of the overcoating and undercoating inks. Theovercoating ink and the undercoating ink were each ejected onto arecording medium (PET50A) at 720 dpi by 720 dpi and 10 ng/dot to form asolid image so that the resulting printed article (recorded article)would have a thickness of 7 to 8 μm.

Then, the solid image was irradiated with UV light having a peakwavelength of 395 nm and an irradiation intensity of 1 W/cm² from aUV-LED in a UV irradiation apparatus disposed at a side of the carriageat irradiation energies gradually increased to 200 mJ/cm² for a pass.Thus, a recorded article was produced. When the surface of the coatingfilm became unsticky during irradiation, the ink was considered to havebeen cured at that time.

Whether or not the coating film was sticky was determined from thefollowing. It was determined according to whether the ink was attachedto a cotton swab or whether the solid image was scratched. When the inkwas not attached to a cotton swab while the solid image was notscratched, the film coating was determined to be unsticky. For thisevaluation, Johnson swab manufactured by Johnson & Johnson was used asthe cotton swab. The solid image was reciprocally rubbed ten times at aload of 100 g.

The number of passes refers to the number of UV irradiation operationsperformed by moving the head to the recording medium and irradiating thecoating film from a UV irradiation apparatus mounted to the head.Irradiation energy (mJ/cm²) were calculated as a product of theirradiation intensity (mW/cm²) at a surface irradiated from the lightsource and the time (s) for which the irradiation had been continued.Irradiation intensity was measured with a UV intensity meter UM-10 and alight receiver UM-400 (each produced by Konica Minolta Sensing, Inc.)The evaluation criteria were as follows, and the results are shown inTable 3 and 4:

-   -   A: The solid image was cured at an irradiation energy of 200        mJ/cm² or less.    -   B: The solid image was not cured at an irradiation energy of 200        mJ/cm².

TABLE 3 Example 1 Example 2 Example 3 Example 4 Example 5 Example 6Example 7 Overcoating ink No. Overcoat 1 Overcoat 1 Overcoat 1 Overcoat2 Overcoat 1 Overcoat 1 Overcoat 4 Undercoating ink No. Undercoat 1Undercoat 2 Undercoat 3 Undercoat 1 Undercoat 7 Undercoat 5 Undercoat 1Surfactant in overcoating Silicone Silicone Silicone Silicone SiliconeSilicone Silicone ink UV 3500 UV 3500 UV 3500 UV 3500 UV 3500 UV 3500 UV3570 Surfactant in undercoating Acrylic Acrylic — Acrylic AcrylicAcrylic Acrylic ink BYK 350 BYK 350 BYK 350 BYK 350 BYK 381 BYK 350Undercoating ink γ − 10 5 15 10 10 10 8 overcoating ink γ VEEA (mass %)in 20 20 20 0 20 20 20 overcoating ink VEEA (mass %) in 30 30 30 30 7030 30 undercoating ink Wetting/spreading A B A A A A B properties ofovercoating ink Burying property of A A B A A A A undercoating ink onrecording medium Ejection stability of A A A A A A A overcoating inkEjection stability of A A A A A B A undercoating ink Curability ofovercoating A A A B A A A ink Curing ability of A A A A A A Aundercoating ink

TABLE 4 Comparative Comparative Comparative Comparative ComparativeExample 1 Example 2 Example 3 Example 4 Example 5 Overcoating ink No.Overcoat 1 Overcoat 1 Overcoat 3 Overcoat 3 Overcoat 2 Undercoating inkNo. Undercoat 6 Undercoat 4 Undercoat 1 Undercoat 4 Undercoat 6Surfactant in overcoating ink Silicone Silicone Acrylic Acrylic SiliconeUV 3500 UV 3500 BYK 350 BYK 350 UV 3500 Surfactant in undercoating inkAcrylic Silicone Acrylic Silicone Acrylic BYK 350 UV 3500 BYK 350 UV3500 BYK 350 Undercoating ink γ - overcoating ink γ 10 0 0 −10 10 VEEA(mass %) in overcoating ink 20 20 20 20 0 VEEA (mass %) in undercoatingink 0 30 30 30 0 Wetting/spreading properties of C C C D C overcoatingink Burying property of undercoating ink B A A A B on recording mediumEjection stability of overcoating ink A A A A A Ejection stability ofundercoating ink A A A A A Curability of overcoating ink A A A A BCuring ability of undercoating ink B A A A B

The results of the above evaluations show that the ink sets of theExamples, each including an undercoating ink containing a (meth)acrylicacid ester having a vinyl ether group expressed by general formula (I)and an overcoating ink having a lower surface tension than theundercoating film, exhibited superior properties to the ink sets ofComparative Examples. Specifically, the overcoating ink exhibitedsuperior wetting/spreading property, the overcoating and undercoatinginks exhibited high curability and ejection stability, and that theundercoating ink exhibited satisfactory burying property on therecording medium.

Additional examination, but not shown in the Tables, was performed usingan undercoating ink prepared in the same manner as undercoating ink 3except that the DPGDA content was set to 26.75% by mass and 0.05% bymass of UV 3500 was added as a surfactant, and overcoating inks 1 and 4.The results were as follows: the burying property of the undercoatingink on the recording medium was B; ejection stability of theundercoating ink was A; the curability of the undercoating ink was A;the differences in surface tension between the undercoating ink and theovercoating ink (γ of undercoating ink−γ of overcoating ink) were each 5mN/m; and the wetting/spreading properties of the overcoating inks wereeach B.

The entire disclosure of Japanese Patent Application No. 2012-043261,filed Feb. 29, 2012 is expressly incorporated by reference herein.

What is claimed is:
 1. A photo-curable ink jet ink set comprising: awhite photo-curable ink jet ink containing a (meth)acrylic acid esterhaving a vinyl ether group expressed by general formula (I):CH₂═CR¹—COOR₂—O—CH═CHR₃, wherein R¹ represents a hydrogen atom or amethyl group, R² represents a divalent organic residue having a carbonnumber of 2 to 20, and R³ represents a hydrogen atom or a monovalentorganic residue having a carbon number of 1 to 11; a color photo-curableink jet ink containing a phenoxyethyl (meth)acrylate; and wherein thecolor photo-curable ink jet ink is either of a black ink, or a yellowink, or a magenta ink, or a cyan ink; wherein the white photo-curableink jet ink and the color photo-curable ink jet ink contain anacylphosphine oxide compound as a photo-radical polymerizationinitiator, and the white photo-curable ink jet ink and the colorphoto-curable ink jet ink are photo-radical polymerization photo-curableinks.
 2. The photo-curable ink jet ink set according to claim 1, whereinthe white photo-curable ink jet ink contains a bifunctional(meth)acrylate.
 3. The photo-curable ink jet ink set according to claim1, wherein a content of the acylphosphine oxide compound of the whitephoto-curable ink jet ink and the color photo-curable ink jet ink areeach 20% by mass or less relative to the total mass of the ink.
 4. Thephoto-curable ink jet ink set according to claim 1, wherein theacylphosphine oxide compound contains a2,4,6-trimethylbenzoyldiphenylphosphine oxide.
 5. The photo-curable inkjet ink set according to claim 1, wherein the white photo-curable inkjet ink contains a phenoxyethyl (meth)acrylate.
 6. The photo-curable inkjet ink set according to claim 1, wherein the color photo-curable inkjet ink contains a tetrahydrofurfuryl (meth)acrylate.
 7. Thephoto-curable ink jet ink set according to claim 1, wherein the whiteink is used as an undercoating photo-curable ink jet ink; and whereinthe color ink is used as an overcoating photo-curable ink jet ink.